The present disclosure relates to an organic EL (electroluminescence) display device and a method for production thereof, the organic EL display device emitting light based on the principle of organic electroluminescence.
The recent rapid development in the information communications industry has aroused the need for high-performance display devices of next generation. One product expected to meet this need is an organic EL element of spontaneous emission type, which has a wide view angle, a high contrast, and a short response time.
The organic EL element has emitting layers and other layers which are made of either low-molecular weight materials or polymeric materials (high-molecular weight materials). The former is known to exhibit a higher emission efficiency and a longer life than the latter in most cases.
The organic film as a constituent of the organic EL element is formed by dry process or wet process. The dry process, which is applicable to low-molecular weight materials, includes vacuum vapor deposition or the like. The wet process, which is applicable to polymeric materials, include coating, such as spin coating, ink jet coating, and nozzle coating, and printing, such as flexographic printing and offset printing.
The vacuum vapor deposition has the advantage of requiring no step for dissolving the material for organic thin films in a solvent and also requiring no step for solvent removal after film formation. Unfortunately, this advantage is offset by difficulties involved in patterning through a metal mask and by high cost for production equipment. Such difficulties and high production cost are detrimental to large-sized panels. In other words, the vacuum vapor deposition presents difficulties in application to large substrates for mass production. One promising way to tackle this problem is the coating method which is easily applied to the production of large display panels.
The coating method typified by the ink jet coating was originally intended to form a laminate structure composed of a hole-injecting layer, an emitting layer, and a cathode electrode which are placed one over another. The hole-injecting layer is formed from such materials as poly-3,4-ethylenedioxythiophene (PEDOP) and polystyrenesulfonic acid (PSS). The emitting layer is formed by coating from materials capable of emitting light in various colors. The cathode electrode is formed from aluminum or the like, with alkali metal interposed thereunder. This original laminate structure has subsequently been improved by inserting an additional layer (called interlayer) made of a polymeric material. The interlayer in general use is intended to keep out deleterious factors from PEDOT and PSS and to control hole injection. In other words, this improved laminate structure is composed of anode electrode, hole transporting layer, interlayer, light-emitting layer, electron injecting layer, and cathode electrode. The first four layers were formed ordinarily by coating and the last two layers, by vapor deposition.
The existing laminate structure mentioned above has problems arising from injecting electrons into the emitting layer from the cathode electrode directly through the electron injecting layer. The first problem is quenching due to direct contact between the emitting layer and the cathode electrode. To avoid quenching, it is necessary to make the emitting layer thick so that the actual emitting position is a certain distance away from the cathode electrode. This results in an increase in drive voltage. The second problem is lack of uniformity in emission efficiency and life for all the colors. (This harms the device characteristic properties.) The reason for this is explained below. Direct injection of electrons from the cathode electrode into the emitting layer is controlled by the electron injecting layer which is made mainly of any of alkali metals and alkaline earth metals and inorganic compounds thereof, such as LiF, MgF2, NaF, MgAg, MgO, Li2O2, SrF2, Ca, Ba, BaO, and Cs. In the case of a display producing three colors (red, green, and blue), one of the foregoing materials is selected to form the layer in common for all the three colors because it is difficult to form the electron injecting layers separately with different colors. Unfortunately, the optimal material differs depending on the color to which it is applied, and this makes it impossible for one material to perform best for all the three colors.
An organic EL element was developed to address this problem, as disclosed in Japanese Patent Laid-Open No. 2006-344869 (hereinafter referred to as Patent Document 1). It is composed of a cathode electrode, an electron injecting layer, an electron transporting layer, and a light emitting layer. The electron transporting layer is formed by vacuum deposition from a material capable of transporting electrons. New to this organic EL element is the addition of the electron injecting layer made of one of the above-mentioned materials placed between the electron transporting layer and the cathode electrode.